In sub-micron scale integrated circuits, CMP techniques are used to create the planarity required in multi-level interconnect structures. Specifically, to create a planar surface for depositing an interconnect layer, e.g. aluminum, tungsten, or copper, an interlayer dielectric (e.g., silicon dioxide) is planarized by a polishing process. This polishing process uses a polishing pad, usually polyurethane, under pressure in frictional contact with the wafer surface. The polishing pad carries an alkaline or acidic slurry with a fine abrasive.
CMP in semiconductor processing removes the highest points from the surface of a wafer to polish the surface, as described for example in Leach, U.S. Pat. No. 5,607,341, issued Mar. 4, 1997. CMP operations are performed on unprocessed and partially processed wafers. A typical unprocessed wafer is crystalline silicon or another semiconductor material that is formed into a nearly circular flat wafer. A typical wafer, when ready for polishing, has a top layer of a dielectric material such as glass, silicon dioxide, or of a metal conformally overlying one or more patterned layers. These underlying patterned layers create local protrusions on the order of about 1 .mu.m in height on the dielectric surface of the wafer. Polishing smoothes the local features, so that ideally the surface of the wafer is flat or planarized over an area the size of a die (a potential semiconductor chip) formed on the wafer. Currently, polishing is sought that locally planarizes the wafer to a tolerance of about 0.3 .mu.m over the area of a die about 10 mm by 10 mm in size.
To maintain uniformity over the polished surface of the interlayer dielectric and to provide wafer-to-wafer reproducibility (global uniformity) of the polishing process, the polishing surface, typically a polyurethane pad, is required to be conditioned during use or between uses.
Polishing rate and uniformity depend in a complex fashion on a number of process variables at the wafer-pad interface, significantly contact pressure, relative velocity between the polishing pad and wafer surface, hardness (durometer) of the polishing pad, properties of the slurry, and rate of chemical reaction. Many of these variables are temperature dependent, particularly the chemical reaction rate, although the polishing pad durometer and slurry viscosity, for example, are also temperature dependent.
Because of the temperature dependence of process variables in CMP, it is desired to regulate the temperature in order to stabilize these process variables. It is additionally desired to provide precise control of temperature over the range of interest, i.e., a range of about 40 degrees F. to 120 degrees F. (about 4.degree. C. to about 50.degree. C.). It is ultimately desired to provide a controlled distribution of temperature locally across a wafer surface and from wafer-to-wafer.
Traditionally, CMP is performed using a planetary CMP apparatus. FIG. 1 is a schematic plan view of a planetary CMP apparatus 100. As shown in FIG. 1, CMP apparatus 100 includes a polishing table or platen 103, rotating in the direction indicated by reference numeral 105. Onto platen 103 is mounted a polishing pad 104. A silicon wafer (not shown) is mounted onto a polishing head 101 and is pressed against the surface of polishing pad 104. Polishing head 101 rotates the silicon wafer in a direction 109, generally in the same direction 105 of rotating platen 103. In addition, an oscillating arm 106 reciprocates polishing head 101 laterally along an arc indicated by reference numerals 108a and 108b. Correspondingly, a conditioning pad (not shown) is mounted onto a smaller platen 102 against polishing pad 104. Platen 102 rotates in the direction indicated by reference numeral 110 and is reciprocated by an oscillating arm 111 along an arc indicated by reference numerals 107a and 107b throughout the CMP process. Slurry is sprayed or otherwise applied onto the surface of polishing pad 104 by a slurry dispenser 113 throughout the CMP process.
Temperature regulation is difficult to achieve in the traditional planetary CMP configuration of FIG. 1. Non-uniform heating is produced by friction at the wafer-pad interface, due to the locally variable and complex motion of the polishing pad relative to the wafer surface. Temperature stabilization has been attempted by passing temperature controlled water or other heat transfer fluid through passages (not shown) internal to platen 103. Fluid temperatures have typically ranged from about 4.degree. C. to about 50.degree. C. Internal fluid cooling requires complicated rotary fluid feedthroughs. Additionally, platen 103 has a large thermal mass, and therefore causes substantial thermal hysteresis. Further, it is difficult to transfer heat between rotating platen 103 and the wafer-pad interface with a distribution that offsets the frictional heating profile.
Temperature stabilization of a CMP process has also been attempted by cooling or heating the slurry prior to dispensing by slurry dispenser 113. As with platen temperature stabilization, the result has been at best to bias the average process temperature lower or higher. This temperature bias can increase or decrease the removal rate globally, but cannot offset the complex local non-uniform temperature profile generated by frictional heating at the wafer-pad interface.
Thus, attempts at temperature regulation in a traditional planetary CMP process have produced unsatisfactory results. What is needed in the art is an apparatus and method to regulate the temperature in order to stabilize a CMP process. Additionally needed are an apparatus and method to provide precise control of temperature in a CMP process over the temperature range of interest, typically a range of about 4.degree. C. to 50.degree. C. Ultimately needed in the art are an apparatus and method to provide a controlled local temperature distribution across a wafer surface and from wafer-to-wafer in a CMP process. The above needs should be fulfilled without incurring excessive cost, complexity, or detrimental side effects such as thermal stresses.